Destroyer GC # 120

[151AND8TH]

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Destroyer # 120
The answer is B: 52 kj

For those types of problem it says dH= reactant - product
So I assuming you use [reactant - product] when dealing with heat of FORMATION?

but I got the answer using [product - reactant] although it came out to be a positive answer


My question is-- when exactly do we use [product - reactant] vs [reactant - product]?

Thanks

 

[D.D.S.]

They are using [bonds broken - bonds formed] not [reactants - products]. You use [products - reactants] when they give you heats of formation, and [bonds broken - bonds formed] when they give you bond energies. I dont think you use [reactants - products] in anything... correct me if im wrong.

hope this helps.

 

[TheWiredNerv]

Yea I messed this one up too but then thanks to the help of SDN I realized the errors in my ways. For these types of problems it is Reactants - Products. The reactants have their bonds broken and thus the products form the new bonds. This translates to Reactants - Products.
An extra word of advice for these problems. You don't have to calculate every single bond in the compounds. Just include the bonds that were broken and formed. For example if you have C6H14 with Br2 and you form C6H13Br and HBr the only bonds broken is one C-H bond and one Br-Br bond and the only bonds formed are 1 C-Br bond and 1 H-br bond. No need to calculate all the other bonds as they will just drop out in the subtraction.

 

[151AND8TH]

Thanks a lot for the clarification guys...

to add to DDS response -- I realize that the table given within the problem is a BOND ENERGY-- KJ/MOL table as oppose to a dH table so that would be one clear indication to which method to use :[bondbroken-bondmade] or the [product-reactant]


please correct me if im wrong..looking over some Gchem notes.. i think you use the [reactant-product] when determining bond enthalpy (dH).. and I think that would be the only situation that would be used. example would be to find the bond enthalpy of the oxidation of glucose